Certain types of printing systems are adapted for printing images on large-scale print media, such as for museum displays, billboards, sails, bus boards, and banners. Some of these systems use so-called drop on demand ink jet printing. In these systems, a piezoelectric vibrator applies pressure to an ink reservoir of the print head to force the ink out through the nozzle orifices positioned on the underside of the print heads. A set of print heads are typically arranged in a row along a single axis within a print head carriage. As the carriage scans back and forth along the direction of the print head axis, the print heads deposit ink across the width of the substrate. A particular image is created by controlling the order at which ink is ejected from the various nozzle orifices.
Some of these systems use inks with different colors to create the desired image. For instance, black, yellow, cyan, and magenta colored inks are commonly employed alone or in combination to generate the image. Thus, combinations of these four colors are used to create various other colors. For instance, a green region of the image is produced by depositing a yellow layer of ink and a cyan layer of ink.
The inks used in these systems are typically “subtractive”-type inks, meaning that as ambient (i.e., white) light passes through the image, each different ink, or combination of inks, “subtracts” light of certain characteristic wavelengths, so that an observer views each respective ink or combination of inks on the substrate as having a particular color (e.g., yellow, cyan, magenta, etc.). Because of this, it is generally required that the images to be printed on a white or near-white background—such as on a white substrate—to assure that an observer will see the proper colors in the final printed image. Otherwise, colors from behind the ink pattern may interfere with the colors of the inks and distort the image seen by the observer.
Accordingly, in order to print color images on non-white substrates, such as colored or transparent substrates, it is typically necessary to deposit a layer of white ink to serve as a backdrop for the color inks. For instance, to print a multi-colored image on a black or colored substrate, the area of the substrate on which the image is to be printed is first pre-coated with a layer of white ink, and then the image is printed on top of the white pre-coat layer. The white background layer prevents the colors in the image from being distorted by the black or colored substrate.
When printing on a transparent substrate, the colored inks are typically applied on the reverse side of the substrate, so that the image may be viewed through the front side of the substrate. Then, a layer of white ink is printed over the colored ink pattern in what is known as a “post-coating” step. The white “post coat” layer serves as a backdrop so that the colors of the image appear properly when viewed from the front side of the transparent substrate. Typically, the transparent substrate is then laminated onto a second transparent substrate, such as a window, so that the color image is protected between the two transparent substrates.
One drawback to the existing techniques for ink-jet printing on non-white substrates is that they require a separate “pre-coating” or “post-coating” step. These additional steps may be performed on a separate printing system configured to print a layer of white ink, but this is an extremely time-consuming and costly solution. Alternatively, there are some ink jet printing systems that are capable of performing the “pre-coating” or “post-coating” steps by providing a pair of separate, dedicated print heads for printing white ink onto the substrate. One example of such a system is the Rho 160W printer from Durst Phototechnik AG, of Brixen, Italy. In these systems, dedicated print heads are located adjacent to the leading and trailing edges of the main print head array for depositing a layer of white ink onto the substrate either prior to, or subsequent to, the main printing operation. An example of this type of printing system is shown schematically in FIG. 3. One disadvantage to this type of system is that the print head carriage must be made larger to accommodate the dedicated pre-coat and post-coat print heads, which are located outside of the main axis of colored ink print heads. Also, these extra print heads are relatively expensive, and may add significant costs to the printing system.